JWST turns up even more heavier-than-expected black holes

These young supermassive black holes weigh more relative to their host galaxies than those today, supporting an idea called heavy seeding.
By | Published: January 24, 2024

In the two years since the James Webb Space Telescope (JWST) launched, astronomers are learning one thing: The early universe is a lot weirder than we thought.

JWST, which is the largest space telescope ever launched, is able to see back in time like never before thanks to a combination of its large mirrors and its sensitivity to infrared light. This means that it can see in stunning clarity objects whose light was emitted billions of years ago in the ultraviolet or visible portion of the spectrum, and has now been stretched by the universe’s expansion to longer wavelengths than optical telescopes like Hubble can pick up.

Astronomers are using it to peer back to near “cosmic dawn,” a time when the first stars and galaxies were forming. And JWST is showing that these early galaxies are different than astronomers had anticipated, in a plethora of ways: Some are settling into shapes we didn’t think were possible so early after the Big Bang. Others are unexpectedly large.

And recent research shows that even the black holes in the early universe were odd — they’re way bigger than they should be, relative to the mass of the galaxy around them. Unexpectedly, JWST is spotting mammoth black holes anchoring relatively small galaxies.

Starting too large

“We know in the local universe there is a supermassive black hole at the center of every massive galaxy at least,” Fabio Pacucci, a Center for Astrophysics | Harvard & Smithsonian researcher, says. These supermassive black holes range from millions to billions of times the mass of the Sun. “And there are pretty tight correlations between the mass of the black hole and many properties of the host galaxy.”

But according to Pacucci and colleagues’ research, published in The Astrophysical Journal Letters and presented at the 243rd meeting of the American Astronomical Society, that correlation seems to break down in the early universe. While now, “big black holes are in big galaxies, small black holes are in small galaxies,” as he says, black holes in the early universe might be 10 to 100 times larger than their counterparts, relatively speaking.

Pacucci says typically, a supermassive black hole at the center of a galaxy now is about 0.1 percent the total mass of the galaxy. But galaxies at high redshifts — i.e., early times in the universe’s history, such as those in this study between 700 million and 1.5 billion years after the Big Bang — have black holes often between 1 percent and 10 percent the mass of their host galaxy.

While the astronomical community had seen hints of such a trend, Pacucci’s study looks at as many high-redshift galaxies as possible from JWST to get a sense of how big the black holes were back then. “And you could clearly see … that every single supermassive black hole studied by JWST up to that point was very over-massive with respect to stellar mass of the galaxies,” he says. Subsequent checks on the data held up, so what astronomers saw seemed to be real and not an observational bias.

How to grow a black hole

So why are the black holes so big so early? Pacucci says it may be due to a process called heavy seeding.

First, some black hole taxonomy. Black holes are sorted into three different categories. Stellar-mass black holes are between four and 100 times the mass of the Sun. These form when massive stars explode as supernovae, leaving behind a core that collapses into a black hole. Next are intermediate-mass black holes, between 100 and 100,000 times the mass of the Sun. They likely form from the mergers of many smaller stellar-mass black holes. Finally, supermassive black holes are at the center of large galaxies. Astronomers aren’t sure how these form, particularly those early in the universe, when there wasn’t yet time to build them up from mergers.

One possibility is that the black hole “seeds” that would ultimately become supermassive black holes formed similarly to stellar-mass black holes today. Because the first population of stars was much more massive than any stars today, these would have left behind larger black holes — perhaps even directly forming intermediate-mass black holes. Supermassive black holes, then, would form from these seeds, further accumulating mass and merging over time.

But heavy seeding is different. In the early universe, there could have been gas clouds that grew so huge and so dense they couldn’t even form into stars — they just collapsed right into black holes 10,000 times the mass of the Sun or more. Heavy seeding can create small supermassive black holes from the outset, rather than large intermediate-mass black holes.

A galaxy with such a black hole in the center would have found star formation difficult, if not impossible — which makes heavy seeding an even more likely culprit for the galaxies Pacucci’s team has seen. “The over-massive black hole is really injecting a lot of energy into the system,” Pacucci says. “It’s heating up the gas. And once the gas is heated up, it’s very hard to form stars.”

Pacucci says it likely wasn’t until about 2 billion or 3 billion years after the Big Bang that things began to settle down and the material outside the black hole started to take on a larger bulk of the mass of the galaxy.

Subsequent work could help piece together the steps between massive seeds and more massive galaxies, as well as whether the study’s finds are truly representative of the conditions of the early universe. Additionally, “we are studying what are the consequences of this for black hole seeding, meaning that we want to better understand how and if this result is really conclusive in saying that black holes form heavy in the high-redshift universe,” Pacucci says.